The applications of thermal recording are expanding in the fields of facsimile telegraphs and printers, labels including POS, etc., because this recording technique has the following advantages: (1) wet development is unnecessary, (2) the quality of paper supports for thermal recording materials is similar to that of general paper, (3) handling is easy, (4) developed-color densities are high, (5) the recording apparatus is simple and inexpensive, and (6) the recording operation is noiseless.
Under these circumstances, the development in recent years of a transparent heat-sensitive recording material for directly recording images with a thermal head has been desired in order to satisfy the demand for multicolor recording or to provide recorded images which can be projected with an overhead projector or directly observed on a light table.
Among recent proposals is a transparent heat-sensitive recording material comprising a transparent support, e.g., a synthetic polymer film, having formed thereon a heat-sensitive recording layer containing a substantially colorless color-forming ingredient A and a substantially colorless color-forming ingredient B which develops a color upon reacting with the color-forming ingredient A. These ingredients A and B are incorporated into the heat-sensitive recording layer by dispersing fine particles of both ingredients into a binder or by microencapsulating one of the ingredients and using the other in the form of an emulsion.
Such heat-sensitive recording materials, although having satisfactory transparency, are disadvantageous in that they are prone to stick or generate noise when printed with a thermal recording device, e.g., a thermal printer. To overcome this problem, a technique of coating the heat-sensitive recording layer with a protective layer mainly comprising a pigment and a binder has been proposed.
However, in order to reduce the sticking and noise generation to a satisfactory level by means of a protective layer mainly comprising a pigment and a binder, the proportion of the pigment ingredient in the protective layer is preferably increased to a value equal to or larger than that of the binder ingredient contained in the protective layer. However, such a high pigment proportion may disadvantageously impair transparency. In conventional opaque heat-sensitive recording materials, an increased pigment proportion in the protective layer can also cause problems. For example, the recorded images thereby obtained are blurry and the developed-color density is decreased.